This invention relates to a method and apparatus for suppressing digital error noise contained in the received data signals in digital telecommunication. Pulse code modulation (PCM) communication, a means of communication relatively free from the effect of noise, has found the widest acceptance among the various modes of digital telecommunication. It not merely provides high-quality communication but also makes possible communication having a large channel capacity by means of high-density and multiplex transmission. It has already been extensively adopted in communications satellites and telephone systems. PCM communication is accomplished on the transmitter side by sampling the information signals at high speed to generate sampled values, encoding these sampled values in amplitude into serial binary signals and transmitting the binary signals using a carrier wave, and on the receiver side by receiving and decoding the binary signals to reproduce the information. The binary signals in this system of a fixed amplitude are serially arranged in correspondance with the amplitude of the sampled values, i.e., the arrangement of the binary signals represent the amplitude of the respective sampled values and are required for reproducing the transmitted information. This means that the transmitted information is unrelated to the amplitudes of the sampled values and that noise below a predetermined amplitude level of the binary signal has no effect whatever upon the information in transmission.
When the information transmitted via the propagation path is exposed to a noise of a level high enough to deform the binary signal owing to atmospherics, electric disturbance, rain-fall attenuation, fading and so on, the signals are no longer correctly decoded on the receiver side. Still more, a loud impulse noise is produced if a binary signal having a charge of relatively large value in one sampling block is damaged by noise to modify the sign of the binary signal. In actual communication systems, noise of this kind occurs rather infrequently and discretely; nevertheless, it is highly desirable for such a noise to be completely absent from the final output of the receiver. Many techniques have conventionally been proposed with a view to suppressing noise caused by digital error.
The most general method for suppressing noise due to digital error comprises transmitting data signals with independent control signals for correction of digital error being intruded into the data signals and comparing on the receiver side the received data signals with the received control signals to detect and correct digital error, if any. Although this method enables detection and correction of digital error with high probability, it not only necessitates installation of at least two independent processing systems each on the transmitter and receiver but also entails inevitable use of two independent communication bands. This method therefore proves inconvenient in the respect that it will make it difficult to secure sufficient communication lines for the large increase in transmission volumes expected in the future.
Another noise-suppressing method which diminishes or eliminates undesirable effects of electrical disturbances when the carrier level is increased beyond the noise level as in the frequency modulation (FM) receiver has suffered from the disadvantage that spectral distortion occurs in the processed signals and degrades the quality of the signals.